I'm afraid that if any completely open mobile device is released that the support from the parent company will be gone very quickly. I think so because: the die hard nerds will be able to make their own distro (removing a very vocal lobby group), and also because the company will think that the free open source community will provide for upgrades, so they won't have too.

This will cause a lot of different distros with compatibility problems. This will make it much more difficult for newbies to get started and getting the most out of the system. The system will stay a nerd gadget.

All the different distros are fine on the PC because there are a lot of systems, and they are (mostly) compatible. On a mobile device that sells limited numbers such a diversification will kill community support because there will be too little user for each distro.

A mobile open source device needs a certain amount of support from a parent company and a certain amount of community developers in order to succeed.
And I don't think opening up a device completely will increase either.